Method and apparatus for determining the size of defects in rolling element bearings with high frequency capability

ABSTRACT

A method and apparatus for determining the length of bearing defects along the direction of rolling, including also a transducer system with high frequency (10,000 HZ) capability for detecting and measuring the displacement and/or vibration of objects placed in contact therewith. The system includes a reflective cantilever spring which serves as the target for reflecting incident light and vibrating in unison with a contacted object. Fiber optics guide light from a light source to the target and back to a signal generator. A crowned cylindrical ruby tip provides the ojbect contacting surface and is biased by the cantilever spring. The light guiding fiber optics and cantilever are sealed from the hostile environment of the transducer.

This application is a division of application Ser. No. 011,603, filedFeb. 6, 1987, now U.S. Pat. No. 4,814,603 which is acontinuation-in-part of application Ser. Nos. 748,084 filed June 24,1985 and 886,827, filed July 18, 1986, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to motion transducers, and more particulary totransducers for measuring displacement and very small amplitude highfrequency vibrations in hostile environmental conditions such as grease,oil, metallic sludge, corrosion, high ambient vibration, hightemperature, electrical, and electromagnetic interference.

2. Related Art

Fiber optic devices for the detection and measurement of displacementand vibration have been disclosed by U.S. Pat. Nos. 3,273,447 to Frankand by 3,327,584 to Kissinger. Those devices have the capability toprovide displacement masurements over a wide frequency range, includingthe range 0-10,000 Hz. However, the output of those devices attributableto Kissinger, which have been commerically marketed, are proportional totarget surface motion as well as target surface reflectivity. To senseand measure motion precisely with these devices it is necessary toensure that the target surface reflectivity is constant whilemeasurements are being taken.

It has been found that accurate dynamic measurements can not be madewith unencapsulated fiber optic devices in environments where there iscontamination of the target surface or of the optical path to the targetsurface. Other non-contact motion transducers, such as eddy current orcapacitive types can also provide high frequency displacementmeasurements, but they too suffer a degradation of performance when usedin an environment that causes a metallic-based or any other electricallyconductive contaminant to collect at the sensing tip. For example, whenusing any non-contacting devices to monitor bearing vibration in themanner disclosed in U.S. Pat. No. 4,196,629 to Philips (which is herebyincorporated by reference), it was found that bearings corrode in theirhousings and that the bearing lubricant can migrate into the sensingarea, mixing with the corrosion debris as it migrates. The mixing ofcorrosion products and lubricant creates a metallic-based sludge thatdegrades the performance of any transducer that is sensitive to metallicsubstances or is dependent upon a clear optical path to the target.

Contact probes generally overcome fouling problems however these deviceshave a very limited frequency capability. Dial indicators and linearvariable differential transformers are two examples of contact sensorsthat provide accurate position measurements but can not be used tomeasure vibrations in the displacement domain up to 10,000 hertz.

Miserentino et al., U.S. Pat. No. 4,171,645, disclosed displacementprobes that combined non-contact fiber optic transducers withself-contained contact targets. Miserentino does not provide for highfrequency measurement capability in any of his several embodiments. Infact, it is obvious from his embodiments that only low frequencyvibration or simple position measurements are possible from histeachings.

Sichling et al., U.S. Pat. No. 4,379,226, disclosed an optical sensingdevice which contains a vibrating spring whose frequency of vibration isdetermined by the parameter p to be measured. Sichling does not specifyhow fast the parameter p may vary and it is obvious from the embodimentsgiven that high frequency measurements are not possible with histeachings.

Thalman in U.S. Pat. No. 4,591,712 disclosed a sensing apparatus whereina reciprocal plunger is utilized to alter the amount of light reflectedback into an enclosed bundle of fiber optic elements. Thalman does notprovide for high frequency capability in his device and it is obviousthat his device could not be used for high frequency vibrationmeasurements.

An encapsulated motion transducer has been disclosed in copendingapplication Ser. No. 886,827, filed July 18, 1986, and is designed tooperate in hostile environments with a high frequency capability.

The embodiments submitted in the copending application can be used tomeasure displacement of vibrating objects but there are problems andlimitations with those embodiments. The high frequency response of anyspring-mass system is limited by the first resonant mode of vibration ofthe system. A typical response curve for a spring-mass system is shownin FIG. 1. A successful sensor design is one that operates in the flatregion below the resonant peak and where the resonant peak is above10,000 Hz.

The resonant frequency is proportional to the stiffness of the springand inversely proportional to the mass of the moving elements. Therefor,in the design of a spring-mass system to obtain the highest possibleresonant frequency, the designer should strive to achieve the largestspring stiffness and the smallest mass. In the copending application,the mass of the sapphire tip can not be optimized to extremely smallvalues because the the design requires a ball diameter larger than thediameter of the springs. The spring elements are likewise forced tolarger than optimum values because the fiber optic elements must passthrough the springs in the embodiments shown. The stiffness of thespring elements can not be set at values that are high enough tocompensate for the large masses of the embodiments given. High springstiffnesses cause high contact pressures between the tip and the objectsurface which can result in contact deformations, permanent denting andother problems. In fabricating and testing embodiments shown in thecopending application, the highest resonant frequency that waspractically obtainable was approximately 700 Hz. Other problems such asfriction among the spring elements and between the tip and casing werefound to degrade the performance of devices of the copending applicationby reducing the actual resonant frequency below the value calculatedwhere frictional effects are not considered.

In the device disclosed by U.S. Pat. No. 4,196,629 to Philips vibrationmeasurements are made up to 10,000 Hz. Thus, there is a continuing needin the state-of-the-art for a contact displacement transducer with highfrequency capability to 10,000 Hz.

In U.S. Pat. No. 4,196,629 the outer race of a ball bearing is deflectedoutward radially in the vicinity surrounding each of the balls, and afiber optic proximity probe can be used to detect those deflections.Three types of waveforms are disclosed which result from defects on theouter ring, inner ring, or ball. Also explained is the peak to RMS ratioof the waveforms which could be used as an indicator of impendingbearing failures.

Experience with bearing failures in rotating machines indicates thatdefects on bearing component parts often grow to be of quite significantsize prior to the initiation of catastrophic failure. For example,cracks or spalls initiated on bearing inner or outer rings have beenfound to have grown to the point where they cover the entirecircumference of the ring. There is therefore, a continuing need in thestate of the art of bearing vibration monitoring to be able to determinethe size of bearing defects.

SUMMARY OF THE INVENTION

The invention provides a method for determining the length of bearingdefects along the direction of rolling and a contact transducer formaking vibration measurements in the displacement domain with a highfrequency capability to 10,000 Hz. The transducer sensing means is anon-contact fiber optic bundle whose light throughput is modulated bymotion of a cantilever beam. A ruby contact tip is bonded to the beam.In operation, the ruby tip is biased against a vibrating object byforces from the cantilever beam which has been initially deflected apredetermined amount. Sealing means are provided to protect the sensingmeans from contamination and fouling.

It is a primary objective of this invention to provide a contactdisplacement transducer with a flat frequency response from zero to10,000 Hz.

Another purpose of this invention is to provide an encapsulated fiberoptic contact transducer with a flat frequency response capability fromzero to 10,000 Hz.

Another important purpose of this invention is to provide a highfrequency motion transducer that is not affected by physical environmentor atmospheric problems such as contamination of the sensing path bygaseous, liquid or solid substances.

Yet another purpose of this invention is to provide an encapsulatedmotion transducer having a flat frequency response from DC to any higherfrequency desired which response can be calculated and controlled bydesign.

A transducer system detects and measures the displacement and/orvibration of objects placed in contact therewith. The system includes areflective target for reflecting incident light and vibrating in unisonwith a contacted object. Fiber optics guide light from a light source tothe target and back to a light sensing means. A cylindrical ruby tip isadhesively bonded to a cantilever spring. The ruby tip is crowned toaccommodate misalignment and to minimize contact stresses. The ruby tipis biased against an object surface by the cantilever spring. Thecantilever spring also provides a reflective target for the fiber opticelements. The light guiding fiber optics and cantilever spring aresealed from the hostile environment of the transducer.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention any many of the attendantfeatures thereof will be readily appreciated as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings wherein:

FIG. 1 is a typical response curve of a spring-mass system;

FIG. 2 is a schematic of a first embodiment of the invention;

FIG. 3 shows the target end of the invention in cross-section;

FIG. 4 shows another embodiment of the invention wherein the fiber opticelements are passed through a connector joint;

FIG. 5 shows another embodiment of the invention wherein the tip andspring elements are mounted in a bearing housing and the fiber opticlight guiding elements are removable therefrom;

FIG. 6 is a schematic view of a contact transducer in contact with anundamaged rolling element bearing;

FIG. 6a is a graphic illustration of an undamaged bearing;

FIG. 7 is a block diagram of the circuitry used to display bearingdamage;

FIGS. 8, 9, 10, 11, 12 and 13 are views similar to FIG. 6 illustratingvarious amounts of bearing damage; and

FIGS. 8a, 9a, 10a, 11a, 12a and 13a illustrate the display of thedamaged bearing of FIGS. 8-13.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings, like reference characters designateidentical or corresponding parts throughout the several views.

As stated above FIG. 1 illustrates a typical response curve for aspring-mass system.

Referring to FIG. 2, numeral 1 designates a plurality of fiber opticlight guides which pass into a housing 2 at one end and which arebifurcated at the opposite end into two groups 1a and 1b where a lightsource 3 and a light detector 4 are provided. The light source 3provides continuous illumination of either visible or invisible light tothe fiber optic light guides 1a. The detector 4 is sensitive to theintensity of light that is returned through the fiber optic light guides1b.

Referring to FIG. 3 which shows the sensing end of the invention incross-section, the fiber optic light guides 1 are shown encased in arigid housing 2. A stainless steel cantilever spring 5 protrudes overthe fiber optic elements at a slight angle which is set so that lightreflected into the light guides 1 is at its maximum value. The undersideof the cantilever provides a reflective target for the fiber opticelements and therefore should be large enough to cover the spot of lightsubtended by the fiber optic bundle. The cantilever is electropolishedto maximize reflectivity. In operation, the sensor is brought intocontact with the object such that the cantilever 5 is deflected towardthe fiber bundle 1. The amount of initial deflection should be greaterthan any operational deflection expected to be encountered. A ruby tip 6is bonded to the cantilever using adhesives 7 suitable for thetemperature operating range expected. The adhesives should also beunaffected by any oils or greases or other substances which may comeinto contact with the sensor. The preferred embodiment uses an activatorcured adhesive which has a urethane methacrylate ester base. Structuralepoxy adhesives are also available which will work quite well. The rubytip 6 has a crown radius which should be as large as possible tominimize contact stresses between the ruby tip 6 and a vibrating object9. The radius should not be so large however that misalinement betweentip and object surfaces would cause edge loading of the ruby tip. In thepreferred embodiment, the diameter of the ruby tip is one millimeter andthe crown radius is 2.5 mm. In the preferred embodiment, misalignmentsup to 5 degrees can be tolerated. A flexible seal 10 is made of siliconerubber and is adhesively bonded to the cantilever 5, the fiber bundlehousing 2, and the ruby tip 6 using an adhesive 11 that is suitable forthe operating temperature range to be encountered. This adhesive shouldalso be unaffected by any oils or greases or other substances that maycome into contact with the sealing means. A silicone RTV adhesive isused in the preferred embodiment. An outer ring 12 is provided toenclose the sealing means and to provide support to the cantileverelement. The outer ring 12 is bonded in place with adhesives 13 that aresuitable for the temperature range to be encountered. Structural epoxyadhesives are adequate for this purpose.

In the design of the transducer, the masses of all the moving elements;the ruby tip 6, the spring 5, the adhesives 7 and 11, and the flexibleseal 10 should be considered and kept as small as possible. In thepreferred embodiment, the total of the masses of the moving elementsshould not exceed 0.00004 lbs. The spring rate of the cantilever spring5 should be selected to be large enough to result in a resonantfrequency above 10,000 Hz. In the preferred embodiment, the spring rateshould be at least 200 lb/in. The maximum deflection of the cantileverspring 5 and the resultant contact force between the tip 6 and object 9should be limited to values that give safe contact pressures at thetip/object interface. In the preferred embodiment, maximum cantileverdeflection is 0.008 inch and the maximum contact pressure is 250,000psi.

FIG. 4 illustrates an alternative embodiment of the invention whereinthe fiber bundle 1 is terminated in a connector 15 which contains a pairof fiber optic bundles 16a and 16b. A mating connector 17 contains apair of light guiding fiber optics 18a and 18b. A light source means 20transmits light to the fiber optic guide 18a which couples to the lightguide 16a. Light is reflected back into the fiber optic light guide 16bfrom the cantilever beam 5, coupled to the light guide 18b andtransmitted to a light detector means 21.

FIG. 5 illustrates another alternative embodiment of the inventionwherein the tip 6, cantilever 5 and sealing elements 10 are mounted andfixtured to a bearing housing 25 and wherein a fiber optic light guidemeans 26 can be manually adjusted to set the gap between the fiberbundle 26 and the cantilever beam 5. In this configuration, the fiberoptic light guide could be inserted temporarily into position forrecording of measurements. When measurements are not being taken, thefiber optic light guides can be removed. In that case, a plug cap isinserted in place of the fiber optic bundle 26 to protect the reflectivesurface of the cantilever 5 from contamination.

FIG. 6 shows a contact transducer 30 with high frequency capabilitywhich is in contact with the stationary outer ring of a rolling elementbearing. The rolling element 32 may be a ball or a roller of any type. Alow frequency waveform is shown FIG. 6(a) which is developed by theaction of rolling elements passing by the transducer. The time u is thetime between successive passages of rolling elements. A high frequencywaveform is shown for a typical bearing that is free of defects. Adamage display is shown which is a bar graph having u units along thehorizontal axis. When a bearing has no defect damage, the damage displayshows all bars at approximately the same height.

The damage display operates in accordance with the block diagram shownin FIG. 7. A machine such as an electric motor 40 is monitored with atachometer probe 42, to generate a signal at the shaft rotationalfrequency, and with a bearing motion transducer 44. The waveform fromthe bearing transducer is passed through a band pass filter 46 toseparate out the load dependent deflection component of the bearingsignal. That signal and the shaft tachometer signal are sent to afrequency ratio counter 48 to compute the bearing speed ratio (BSR) asdescribed in U.S. Pat. No. 4,196,629. The bearing motion signal is alsosent through a high pass filter 50 which eliminates the load dependentdeflection data. The signal is then sent to a peak detector 52 whichwill detect spikes in the bearing signal that are caused by damage onthe component parts of the bearing. The output of the peak detector 52is sent to the damage display 54 which is a bar graph display in thepreferred embodiment. The load dependent component of the bearing signalis sent to a period counter 56 which measures the time betweensuccessive roller passages. The BSR, which is the output from thefrequency ratio counter 48, is sent to a processor 58 which puts the BSRvalue into component damage equations for computation of proper triggersignals. The multiplier/divider 60 modifies the period of the bearingload dependent deflection in accordance with values determined by thedamage equations. The trigger signal for the damage display will therebybe perfectly synchronized to the frequency of operation of the bearingcomponent parts.

The processor 58 can also be a human interface whereby the BSR is readand then used to calculate the bearing damage frequencies O for outerring damage, I for inner ring damage, R for roller damage, and C forcage damage in accordance with the formulas O=RPM×BSR, I=RPM×(n-BSR),R=RPM×BSR((OD+ID)/d+2)/n, and C=RPM×BSR/n where RPM is the inner ringrotational speed, n is the number of rolling elements, OD is the outerdiameter of the bearing, ID is the inner diameter of the bearing, and dis the diameter of the rolling element.

A small amount of damage 60 on a bearing outer ring 62 will result inthe waveforms and display shown in FIG. 8(a) where the display time isset to be u which is equal to 1/O. The value of v in FIG. 8(a) willdepend upon the location of the transducer 64 with respect to the defect60. A large damage 66 on a bearing outer ring 68 in FIG. 9 results inthe waveform and display shown in FIG. 9(a). The greater length ofdamage along the outer ring in the direction of rolling causes impactsto occur over a longer time duration w. The size of the damage isdetermined from the ratio w/u where the quantity u is proportional tothe length between two rolling elements along the circumference of theouter raceway.

A small amount of damage 80 on a bearing inner ring 82 in FIG. 10results in the waveform and damage display shown in FIG. 10(a) where thedisplay time x is set to be 1/I. The time 1/I is proportional to thelength between two adjacent rollers along the length of the inner ring.

A large amount of damage 90 on the inner ring 92 in FIG. 11 results inthe waveform and damage display shown in FIG. 11(a). A measure of thelength of the damage is found by computing the ratio y/x. When thisratio equals 1.0, the length of the defect is equal to the lengthbetween rolling elements. It is seen from FIGS. 10(a) and 11(a) that theamplitude of the spikes in the high frequency waveform modulates with aninterval of time z. That interval z is equal to the period of one shaftrevolution. When the ratio y/x approaches 1.0, the damage display shouldbe changed to be synchronized with z. In this manner, the display willindicate the size of the damage as a fraction of the circumference ofthe inner ring.

A small amount of damage 100 on a rolling element 102 in FIG. 12 willresult in the waveforms shown in FIG. 12(a) where the display time isset to be a which is equal to 1/R. As the damage on a roller grows, thewidth of the bar graph will grow proportionately and the size of thedamage can be read as a fraction of the roller semi-diameter.

FIG. 13 shows more than one rolling element 112 with a defect 110. Thehigh frequency waveform and damage display will appear as shown in FIG.13(a) where the display time is set to be b which is equal to 1/C. Inthis mode, a bar will appear for each rolling element that is damaged.

In summary, the invention achieves a high frequency capability to makedisplacement measurements with a contacting device by properly focusingnon-contacting fiber optic sensing means at a reflective cantileverspring that biases a ruby tip against an object whose motion is to bemeasured. By proper design, the masses of the moving elements areminimized and the spring stiffness of the cantilever adjusted formaximum frequency response at safe contact stress levels. In so doing,the advantages of fiber optic displacement sensors namely high frequencyresponse, small size and immunity from electrical and electromagneticnoises are combined with the non-fouling properties of contactingsensors.

It will be understood that variations and modifications may be effectedwithout department from the spirit and scope of the novel concepts ofthis invention. Namely, the material of the cantilever which isstainless steel in the preferred embodiment could likely be berryliumcopper for its good endurance strength, or titanium for its high Young'sModulus, or some composite material to which a reflective coating may ormay not be applied. It is also suggested that contact tips of variousshapes, materials, or configurations can be designed to enhance thereliability, performance, or ease of manufacture of the invention. Forexample, the tip may be mechanically affixed to the spring beam or thebeam itself may be curved or dimpled to form its own contact tip. It isfurther suggested that dynamic motion of the cantilever beam may besensed by means other than light sensing means; for example, eddycurrent or capacitance sensing means may be used.

What is claimed is:
 1. A transducer comprising:(a) means for contactingan object; (b) a cantilever spring for biasing the contacting means tofollow the dynamic motion of the object; (c) means responsive to themovement of the spring for generating a signal commensurate therewith.2. A transducer according to claim 1, where the responsive meansincludes means for reflecting incident light off of said spring.
 3. Atransducer according to claim 2, wherein the light reflecting means iselectropolished stainless steel.
 4. A transducer according to claim 1,wherein the contacting means has an outer peripheral surface which issubstantially arcuate in shape.
 5. A transducer according to claim 1,wherein the contacting means is of a ruby material.
 6. A transduceraccording to claim 1, wherein the contacting means is rigid and asubstantially cylindrical member with one flat end and one crowned end.7. A transducer according to claim 1, wherein the contacting means has adiameter of about 1 mm. and a crown radius of about 2.5 mm.
 8. Atransducer according to claim 1, wherein the biasing means is acantilever spring having a spring rate of at least 200 lb./in.
 9. Atransducer according to claim 1, wherein the biasing means is acantilever spring having a maximum deflection of 0.008 inch and amaximum contact pressure of 250,000 psi.
 10. A transducer according toclaim 2, including sealing means, the sealing means being fixidlyattached to the cantilever spring whereby movement of the lightreflecting means is unaffected by contact with said sealing means.